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742                            Working Valve.                                                  \

Working Valve.—When a D slide is used, Fig. 730 is the                  f

usual form, where p, R, and E are the passages from pressure, to
ram, and to exhaust, respectively. At B the valve is. open to
pressure, and at A to exhaust, while at c the ram passage is
entirely cut off, by hand or automatic gear.                                                «

Hydraulic-pressure Engines, though wasteful with small                   f

pressures and high speeds, may reasonably be used when supplied
with water at 750 Ibs. pressure or more, the piston speed being
not more than 80 ft. per minute.     The first piston engine, in-
vented by the late Lord (then Mr.) Armstrong in 1838, was of the                  !
rotary type.    Subsequently he adopted side-by-side cylinders with
reciprocating pistons, and in the present engine, as applied to                   ^pr
heavy work, such as turning ships' turrets or swing bridges, there                  i
are three oscillating cylinders, whose pistons connect to the same                   I
triple-throw crank shaft, and each valve is worked by a rocking                   \
lever on the trunnion.     Fig. 731 is a section through one valve                     j
box.    Valve A is reciprocated by the trunnion lever, while valve B,                     ,
used for reversing purposes, may for the present be considered
fixed,     c is the pressure supply, D the exhaust pipe, and E F the
connection to the cylinder.   Taking present position of B, a right-
hand movement of A admits pressure to E, and a leftward move-
ment permits exhaust from E, through H and G, to D.    Supposing,
now*, B'S position be so changed that H is opposite D, and G                   \
opposite F ; the conditions are reversed, and a leftward movement                   \
of A admits pressure to F, while a rightward movement exhausts                  ^
through the valve to D.   Thus B is a reversing valve, and is moved
by the piston of an auxiliary cylinder.                                                                   1

The Relief Valve j is simply a small, spring-loaded safety
valve, which permits an escape of water whenever the pressure
exceeds the normal, by reason of water inertia Such valves are
placed wherever there is liability to shock.

The Brotherhood-Hastie hydraulic engine, Fig, 732, is a com-                   \

bination of the well-known Brotherhood engine, p. 632,  with                  y

Hastie's automatic stroke adjustment.    Pressure water entering                  £

at P, passes to the cylinder by pipe A, and the exhaust returns
through the same pipe, but is diverted by valve D into the outlet E.                 /

If p and E are connected to a reversing valve, the pressure water                  |

may enter at E and leave at p, and the direction of engine rotation                  i